Introduction
Networking is the concept of sharing resources and services. A network
of computers is a group of interconnected systems sharing resources and
interacting using a shared communications link. A network, therefore, is a set
of interconnected systems with something to share. The shared resource can be
data, a printer, a fax modem, or a service such as a database or an email
system. The individual systems must be connected through a pathway (called the
transmission medium) that is used to transmit the resource or service between
the computers. All systems on the pathway must follow a set of common
communication rules for data to arrive at its intended destination and for the
sending and receiving systems to understand each other. The rules governing
computer communication are called protocols.
In summary, all networks must have the following:
1.
A resource to share
(resource)
2.
A pathway to
transfer data (transmission medium)
3.
A set of rules
governing how to communicate (protocols)
Figure(1) - Simplest form of a computer network
Having a
transmission pathway does not always guarantee communication. When two entities
communicate, they do not merely exchange information; rather, they must
understand the information they receive from each other. The goal of computer
networking, therefore, is not simply to exchange data but to understand and use
data received from other entities on the network.
An analogy is people
speaking, just because two people can speak, it does not mean they
automatically can understand each other. These two people might speak different
languages or interpret words differently. One person might use sign language,
while the other uses spoken language. As in human communication, even though
you have two entities who "speak," there is no guarantee they will be
able to understand each other. Just because two computers are sharing
resources, it does not necessarily mean they can communicate.
Figure (2) - An analogy of a computer network
Because computers can
be used in different ways and can be located at different distances from each
other, enabling computers to communicate often can be a daunting task that
draws on a wide variety of technologies.
The two main
reasons for using computer networking are to provide services and to reduce
equipment costs. Networks enable computers to share their resources by offering
services to other computers and users on a network. The following are specific
reasons for networking PCs
1.
Sharing files
2.
Sharing printers
and other devices
3.
Enabling
centralized administration and security of the resources within the system.
4.
Supporting network applications such as
electronic mail and database services
5.
Limited resources
6.
Desire to share the
resources
7.
Cost Reduction
Today, that's a limiting view, because the most important resource is
information. Network lets us share information and Resource Sharing achieves
the same.
Resource
Sharing
The purpose of many computer networks is to permit a far-flung community
of users to share computer resources. Many such users now have their own
microcomputers, so the shared resources have to be interesting enough to
warrant access via a network. The facilities accessible by networks are in fact
becoming more interesting at a rapid rate.
The remote computer may contain software that a user
needs to employ. It may be proprietary software kept at one location. It may
require a larger machine than any at the user's location. The distant computer
may provide access to data that is stored and maintained at its location.
Sometimes the remote machine controls a large or special printing facility.
Sometimes the remote machine compiles programs that are used on smaller
peripheral machines.
Cost
Reduction
There are various aspects of technology that are likely to force the
price of terminal usage drastically lower. This is important because almost all
aspects of telecommunications are characterized by high price elasticity. In
other words, when the price comes down, the usage goes up.
Key
Issues For Computer Network
The
following are the major key issues to be trashed out very carefully before we
go for a computer network:
1.
Nature of Nodes -Whether participating nodes are homogeneous or
heterogeneous in nature?
2.
Topology - Which of the computer topology has to be followed?
Computer topology accounts for the physical arrangement of participating
computers in the network.
3.
Interconnection
Type - Whether interconnection
type is point-to-point, multi-point, or broadcast type.
4.
Reliability - How reliable our network is? Reliability aspect
includes error rate, redundancy and recovery procedures.
5.
Channel Capacity
Allocation - Whether allocation of
channel capacity is time-division or frequency division?
6.
Routing Techniques
- Whether message between nodes are to
be routed through: Deterministic, Stochastic, and Distributed routing
techniques?
7.
Models - Which of the models i.e. analytical models, queuing
models, simulation models, measurement and validation models are applicable?
8.
Channel Capacity - What are the channel capacities of the communication
lines connecting nodes?
9.
Access - Whether computer access in the network is
direct-access or through a sub-network?
10.
Protocols - What levels, standards and formats are to be
followed while establishing communication between participating nodes?
11.
Performance - How is higher performance of computer network
achieved? Response time, time to connect, resource utilization, etc. contribute
towards performance of computer network.
12. Control - Whether
centralized control, distributed control or hierarchical control of participating
nodes of computer network is suitable?
Types of
Network- LAN, WAN and MAN
Today when we speak of networks, we are
generally referring to three primary categories: local area networks,
metropolitan area networks, and wide area networks. Into which category a
network its size, its ownership, the distance it covers, and its physical
architecture determine falls.
Figure(3) - Categories of networks
Local
Area Network (LAN)
A local area network (LAN) is usually
privately owned and links the devices in a single office, building, or campus.
Depending on the needs of an organization and the type of technology used, a
LAN can be as simple as two PCs and a printer in someone's home office, or it
can extend throughout a company and include voice, sound, and video
peripherals. Currently, LAN size is limited to a few kilometers.
Figure (4) - LAN
LANs are designed to allow resources to be
shared between personal computers or workstations. The resources to be shared
can include hardware e.g., a printer, software e.g., an application program, or
data. A common example of a LAN, found in many business environments, links a
work group of task-related computers, for example, engineering workstations or
accounting PCs. One of the computers may be given a large-capacity disk drive
and become a server to the other clients. Software can be stored on this
central server and used as needed by the whole group. In this example, the size
of the LAN may be determined by licensing restrictions on the number of users
per copy of software, or by restrictions on the number of users licensed to
access the operating system.
In
addition to size, LANs are distinguished from other types of networks by their
transmission media and topology. In general, a given LAN will use only one type
of transmission medium. The most common LAN topologies are bus, ring, and star.
Traditionally, LANs have data rates in the 4 to 16 Mbps
range. Today, however speeds are increasing and can reach 100 Mbps with gigabit
systems in development.
Metropolitan
Area Network (MAN)
A
metropolitan area network (MAN) is designed to extend over an entire city. It
may be a single network such as a cable television network, or it may be a
means of connecting a number of LANs into a larger network so that resources
may be shared LAN-to-LAN as well as device-to-device. For example, a company
can use a MAN to connect the LANs in all of its offices throughout a city.
Figure(5) - MAN
A
MAN may be wholly owned and operated by a private company, or it may be a
service provided by a public company, such as a local telephone company. Many
telephone companies provide a popular MAN service called Switched Multi-megabit
Data Services (SMDS).
Wide
Area Network (WAN)
A wide area network (WAN) provides long-distance
transmission of data, voice, image, and video information over large
geographical areas that may comprise a country, a continent, or even the whole
world.
Figure(6) - WAN
In contrast to
LANs (which depend on their own hardware for transmission), WANs may utilize
public, leased, or private communication devices, usually in combinations, and
can therefore span an unlimited number of miles. A WAN that is wholly owned and
used by a single company is often referred to as an enterprise network.
Criteria for
Classification of Computer Network
The following are the characteristics used
to classify different types of computer networks
Topology
Topology is nothing but the geometric
management of positioning computer systems to involve them in the form of a
network. For example, Star topology, Bus topology, etc.
Protocol
The protocols are nothing but the set of
rules and signals that are used for communication in the network. For example,
'Ethernet' is one of the most popular protocols for LANs.
Architecture
Networks can usually be classified in the
following two types -
1.
Peer-to-peer
architecture.
2. Client-Server architecture.
The term topology refers to the way a network is laid
out, either physically or logically. Two or more devices connect to a link; two
or more links form a topology. The topology of a network is the geometric
representation of the relationship of all the links and linking devices
(usually called nodes) to each other. There are five basic topologies possible:
mesh, star, tree, bus, and ring.
Figure(7) - Multipoint
Line Configuration
Figure (8) - Categories of Topologies
These five labels describe how the devices in a network
are interconnected rather than their physical arrangement. For example, having
a star topology does not mean that all of the computers in the network must be
placed physically around a hub in a star shape. A consideration when choosing a
topology is the relative status of the devices be linked. Two relationships are
possible: peer-to-peer, where the devices share the link equally, and
primary-secondary, where one device controls traffic and the others must
transmit through it. Ring and mesh topologies are more convenient for
peer-to-peer transmission, while star and tree are more convenient for
primary-secondary, bus topology is equally convenient for either.
Mesh
In a mesh topology, every device has a dedicated
point-to-point link to every other device. The term dedicated means that the
link carries traffic only between the two devices it connects. A fully
connected mesh network therefore has n*(n - l)/2 physical channels to link n
devices. To accommodate that many links, every device on the network must have
7 input/output (I/O) ports.
Figure (9) -
Fully Connected Mesh Topology
A mesh offers several advantages over other network
topologies. First, the use of dedicated links guarantees that each connection
can carry its own data load, thus eliminating the traffic problems that can
occur when links must be shared by multiple devices.
Second, a mesh topology is robust. If one link becomes
unusable, it does not incapacitate the entire system.
Another advantage is privacy or security. When every
message sent travels along dedicated line, only the intended recipient sees it.
Physical boundaries prevent other users from gaining access to messages.
Finally, point-to-point links make fault identification
and fault isolation easy. Traffic can be routed to avoid links with suspected
problems. This facility enables the network manager to discover the precise
location of the fault and aids in finding its cause and solution.
The main disadvantages of a mesh are related to the
amount of cabling and the number of I/O ports required. First, because every
device must be connected to ever other device, installation and reconfiguration
are difficult. Second, the sheer bulk of the wiring can be greater than the
available space (in walls, ceilings, or floors) can accommodate. And, finally,
the hardware required connecting each link (I/O ports and cable can be
prohibitively expensive). For these reasons a mesh topology is usually
implemented in a limited fashion—for example, as a backbone connecting the main
computers of a hybrid network that can include several other topologies.
Star
In a star topology, each device has a dedicated
point-to-point link only to a central controller, usually called a hub. The
devices are not directly linked to each other. Unlike a mesh topology, a star
topology does not allow direct traffic between devices. The controller acts as
an exchange. If one device wants to send data to another, it sends the data to
the controller, which then relays the data to the other connected device.
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Figure (10) -
Star topology
A star topology is less expensive than a mesh topology.
In a star, each device needs only one link and one I/O port to connect it to
any number of others. This factor also makes it easy to install and
reconfigure. Far less cabling needs to be housed, and additions, moves, and
deletions involve only one connection: between that device and the hub.
Other advantages include robustness. If one link fails,
only that link is affected. All other links remain active. This factor also
lends itself to easy fault identification and fault isolation. As long as the
hub is working, it can be used to monitor link problems and bypass defective
links.
However, although a star requires far less cable than a
mesh, each node must be linked to a central hub. For this reason more cabling
is required in a star than in some other topologies (such as tree, ring, or
bus).
Tree
A tree topology is a variation of a star. As in a star,
nodes in a tree are linked to a central hub that controls the traffic to the
network. However, not every device plugs directly into the central hub. The
majority of devices connect to a secondary hub that in turn is connected to the
central hub.
The central hub in the tree is an active hub. An active
hub contains a repeater, which is a hardware device that regenerates the
received bit patterns before sending them out. Repeating strengthens trans-
missions and increases the distance a signal can travel.
Figure (11) - Tree Topology
The secondary hubs may be active or passive hubs. A
passive hub provides a simple physical connection between the attached devices.
The advantages and disadvantages of a tree topology are
generally the same as those of a star. The addition of secondary hubs, however,
brings two further advantages. First, it allows more devices to be attached to
a single central hub and can therefore increase the distance a signal can
travel between devices. Second, it allows the network to isolate and prioritize
communications from different computers. For example, the computers attached to
one secondary hub can be given priority over computers attached to another
secondary hub. In this way, the network designers and operator can guarantee
that time-sensitive data will not have to wait for access to the network.
A good example of tree topology can be seen in cable TV
technology where the main cable from the main office is divided into main
branches and each branch is divided into smaller branches and so on. The hubs
are used when a cable is divided.
Bus
The preceding examples all describe point-to-point
configurations. A bus topology, on the other hand, is multipoint. One long
cable acts as a backbone to link all the devices in the network.
Nodes are connected to the bus cable by drop lines and
taps. A drop line is a connection running between the device and the main
cable. A tap is a connector that either splices into the main cable or
punctures the sheathing of a cable to create a contact with the metallic core.
As a signal travels along the backbone, some of its energy is transformed into
heat. Therefore, it becomes weaker and weaker the farther it has to travel. For
this reason there is a limit on the number of taps a bus can support and on the
distance between those taps.
Advantages of a bus topology include ease of
installation. Backbone cable can be laid along the most efficient path, then
connected to the nodes by drop lines of various lengths. In this way, a bus
uses less cabling than mesh, star, or tree topologies. In a star, for example,
four network devices in the same room require four lengths of cable reaching
all the way to the hub. In a bus, this redundancy is eliminated. Only the
backbone cable stretches through the entire facility. Each drop line has to
reach only as far as the nearest point on the backbone.
Figure (12) - Bus Topology
Disadvantages include difficult reconfiguration and
fault isolation. A bus is usually designed to be optimally efficient at
installation. It can therefore be difficult to add new devices. As mentioned
above, signal reflection at the taps can cause degradation in quality. This
degradation can be controlled by limiting the number and spacing of devices
connected to a given length of cable. Adding new devices may therefore require
modification or replacement of the backbone.
In addition, a fault or break in the bus cable stops
all transmission, even between devices on the same side of the problem. The
damaged area reflects signals back in the direction of origin, creating noise
in both directions.
Ring
In a ring topology, each device has a dedicated
point-to-point line configuration only with the two devices on either side of
it. A signal is passed along the ring in one direction, from device to device,
until it reaches its destination. Each device in the ring incorporates a
repeater. When a device receives a signal intended for another device, its
repeater regenerates the bits and passes them along.
A ring is relatively easy to install and reconfigure.
Each device is linked only to its immediate neighbors (either physically or
logically). To add or delete a device requires moving only two connections. The
only constraints are media and traffic considerations (maximum ring length and
number of devices). In addition, fault isolation is simplified. Generally in a
ring, a signal is circulating at all times. If one device does not receive a
signal within a specified period, it can issue an alarm. The alarm alerts the
network operator to the problem and its location.
However, unidirectional traffic can be a disadvantage.
In a simple ring, a break in the ring (such as a disabled station) can disable
the entire network. This weakness can be solved by using a dual ring or a
switch capable of closing off the break.
Figure (13) - Ring Topology
In computer networks, communication occurs between
entities in different systems. An entity is anything capable of sending or
receiving information. Examples include application programs, file transfer
packages, browsers, database management systems, and electronic mail software.
A system, is a physical object that contains one or more entities, Examples
include computers and terminals. But two entities cannot just send bit streams
to each other and expect to be understood. For communication to occur, the
entities must agree on a protocol. A protocol is a set of rules that govern
data communication. A protocol defines what is communicated, how it is
communicated, and when it is communicated. The key elements of a protocol are
syntax, semantics, and timing.
Syntax
Syntax refers to the structure or format of the data,
meaning the order in which they are presented. For example, a simple protocol
might expect the first eight bits of data to be the address of the sender, the
second eight bits to be the address of the receiver, and the rest of the stream
to be the message itself.
Semantics
Semantics refers to the meaning of each section of
bits. How is a particular pattern to be interpreted, and what action is to be
taken based on that interpretation. For example, does an address identify the
route to be taken or the final destination of the message?
Timing
Timing refers to two characteristics: when data should
be sent and how fast they can be sent. For example, if a sender produces data
at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission
will overload the receiver and data will be largely lost.
In data communication, a protocol is a set of rules
that govern all aspects of information communication.
Protocols
Example
There are a many
standard protocols to choose from, standard protocols have their own advantage
and disadvantage i.e., some are simpler than the others, some are more
reliable, and some are faster.
From a user’s point of view, the only interesting
aspect about protocols is that our computer or device must support the right
ones if we want to communicate with other computers. The protocols can be
implemented either in hardware or in software. Some of the popular protocols
are:
1.
TCP/IP
2.
HTTP
3.
FTP
4.
SMTP
5.
POP
6.
Token-Ring
7.
Ethernet
8.
Xmodem
9.
Kermit
10.
MNP, etc.
To use the services available on an Internet,
application programs, running at two end computers and communicating with each
other, are needed. In other words, in an Internet, the application programs are
the entities that communicate with each other, not the computers or users.
The
application programs using the Internet follow these client-server model
strategies
1.
An application
program, called the client, running on the local machine, requests a service
from another application program, called the server, running on the remote
machine, Figure 2.12 illustrates this.
Figure (13) - Client-server
Model
2.
A server can
provide a service for any client, not just a particular client. In other words,
the client-server relationship is many-to-one. Many clients can use the
services of one server.
3.
Generally, a client
program, which requests a service, should run only when it is needed. The server program, which
provides a service, should run all of the time because it does not know when
its service is needed.
4.
Services needed
frequently and by many users have specific client-server application programs.
For example, we should have client-server application programs that allow users
to access files, send e-mail, and so on. For services that are more customized,
we should have one generic application program that allows users to access the
services available on a remote computer.
Client
A client is a program running on the local machine
requesting service from a server. A client program is finite, which means it is
started by the user (or another application program) and terminates when the
service is complete.
Server
A server is a program running on the remote machine
providing service to the clients. When it starts, it opens the door for
incoming requests from clients, but it never initiates a service until it is
requested to do so.
A server program is an infinite program. When it
starts, it runs infinitely unless a problem arises. It waits for incoming
requests from clients. When a request arrives, it responds to the request.
LANs
A local area network (LAN) is two or more computers
directly linked within a small well-defined area such as a room, building, or
group of closely placed buildings. A LAN may be made up of only microcomputers
or any combination of microcomputers and large systems.
Figure (14) - Local Area Network
The computer network that connects the computers in the
different parts of the same big city like metropolitan city may be referred to
as Metropolitan Area Network (MAN).
Interest in local area networks is constantly growing
due to following two developments
1.
Developments in
communication technology
2.
The difference
between a LAN and Development of powerful and user-friendly micro-computers
A multi-user system is that a LAN is made up of
stand-alone computers whereas a multi-user system typically has one computer
that is shared among two or more terminals.
A LAN usually consists of the following-
1.
Two or more
computers
2.
Peripheral devices
such as printers and hard-disk drives
3.
Software to control
the operation of the computers or other devices connected to the LAN
4.
Special cables,
usually coaxial or fiber optic, to connect the computers and other devices
5.
A plug-in board to
handle the data transmissions.
6.
A benefit of a LAN
is the reduction of hardware costs because several computers and users can
share peripheral devices such as laser printers, hard-disk drives, color
plotters, and modems. Another advantage is the users can share data.
Ensuring the security and privacy of data are two
concerns of LAN users. The LAN must get the data to its destination, transmit
the data correctly, and prevent unauthorized users from gaining access to that
data. These tasks are accomplished through both the hardware and LAN software.
They vary in the type and number of computers that can
be connected, the speed at which data can be transferred, and the type of the
software used to control the network.
Some LANs require that all the computers be of a certain brand, while
others allow a variety of brands to be connected. The number of computers in a
LAN varies widely from smaller LANs that typically connect 2 to 25 computers,
to large LANs that can connect as many as 10,000 computers.
The length of the cable connecting a computer to a LAN
also varies depending on the LAN. Most LANs allow cables of about 1000 feet,
but some allow cables of several miles to be used. The data transfer speeds
range from several thousand bits per second to around 10 million bits per
second. The programs that control the LANs also vary in the features they
offer. Some programs allow the use of more than one operating system; others
allow only one. On some LANs, file access is limited to one user at a time; on
others, more than one user can access a file simultaneously.
Hardware Requirements For LAN
The following are major hardware
components/devices required for establishing LAN
1.
Transmission
Channel
2.
Network Interface
Unit (NIU)
3.
Servers
4.
Workstations
6.
Generally four
types of channels are used for data transmission in a LAN. These are-
i.
Twisted Pair Cable
ii.
Coaxial Cable
iii.
Fiber-Optic Cables
iv.
Radio Waves
Network Interface Unit
Network interface units connect each device in the LAN
network to shared transmission device. It contains the rules or logic to access
the LAN. NIU is also used to implement LAN protocols and for device
attachments. Its function depends on the type of topology used in LAN.
Servers & Workstations
One of the major benefits of implementation of LAN is
sharing expensive resources such as storage devices, printers etc. This is
achieved through providing servers on the LAN. It is dedicated computer that
controls one or more resources. This contains both hardware and software
interface for LAN. Three major categories of services used in LANs are
1.
File Server
2.
Printer Server
3.
Modem Server
In networking, file server is used to share
storage space for files. Besides providing storage space for files in a LAN
environment, it is used for taking periodical backup, and also to provide
gateway to other servers within and between LANs.
Similarly printer server is used to handle printing works
of all workstation connected in the network.
In LAN environment also modem is
required to get connected to other network or simply to use a telephone. A
modem server is used to share this expensive resource by all connected
workstations in a network ring.
LAN operating system is required to operate on the LAN
system. It has basically two aspects
1.
Server Software
2.
Workstation
Software
LAN operating system facilitates
i.
Sharing of
expensive resources e.g. printer, storage space etc.
ii.
Security for data
iii.
Connection to other
network.
There are various types of LAN operating system. Some
popular LAN operating system are-
i.
Novel Netware
ii.
Ethernet
iii.
Curves
iv.
ArcNet
v.
LAN Server
vi.
Omni Net
vii.
PC Net
viii.
IBM PC LAN
ix.
Etherlik Plus, etc.
INTRODUCTION TO ETHERNET
History
of the Ethernet
Ethernet is a well-known and widely used network
technology that employs bus topology. Ethernet was invented at Xerox
Corporation’s Palo Alto
Research Center
in the early 1970s. Digital Equipment Corporation, Intel Corporation, and Xerox
later cooperated to devise a production standard, which is informally called
DIX Ethernet for the initials of the three companies. IEEE now controls
Ethernet standards. In its original version, an Ethernet LAN consisted of a
single coaxial cable, called the ether, to that multiple computers connect.
Engineers use the term segment to refer to the Ethernet coaxial cable. A given
Ethernet segment is limited to 500 meters in length, and the standard requires
a minimum separation of 3 meters between each pair of connections.
The original Ethernet hardware operated al a bandwidth
of 10 Megabits per second (Mbps); a later version known as Fast Ethernet
operates at IUU Mbps. and the most recent version, which is known as Gigabit
Ethernet operates at 1000 Mbps or 1 Gigabit per second (Gbps).
Sharing
on an Ethernet
The Ethernet standard specifies all details, including
the format of frames that computers send across the ether, the voltage to be
used, and the method used to modulate a signal.
Because it uses a bus topology, Ethernet requires
multiple computers to share access to a single medium. A sender transmits a
signal, which propagates from the sender toward both ends of the cable. Figure
1.8 illustrates how data flows across an Ethernet.
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Figure (15) - Conceptual flow of bits across an
Ethernet
A signal propagates from the sending computer to both
end of the shared cable. It is important to understand that sharing in local
area network technologies does not mean that multiple frames are being sent at
the same time. In stead, the sending computer has exclusive use of the entire
cable during the transmission of a given frame- other computers must wait. After one-computer finishers transmitting one
frame, the shared cable becomes available for another computer to use.
Ethernet is a bus, network in which multiple computers
share a single transmits a frame to another, and all other computers must wait.
Carrier
Sense on Multi-Access Networks (CSMA)
The most interesting aspect of Ethernet is the
mechanism used to coordinate transmission.
An Ethernet network does not have a centralized controller that tells
each computer how to take turns using the shard cable. Instead, all computers
attached to an Ethernet participate in a distributed coordination scheme called
Carrier Sense Multiple Access (CSMA). The scheme uses electrical activity on
the cable to determine status. When no computer is sending a frame, the ether
does not contain electrical signals. During frame transmission, however, a
sender transmits electrical signals used to encode bits. Although the signals
differ slightly from the carrier waves, they are informally called a carrier.
Thus, to determine whether the cable is currently being used, a computer can
check for a carrier. If no carrier is present, the computer can transmit a
frame. If a carrier is present, the computer must wait for the sender to finish
before proceeding. Technically, checking for a carrier wave is called carrier
sense, and the idea of using the presence of a signal to determine when to transmit
is called Carrier Sense Multiple Access (CSMA).
Collision
Detection and Back off with CSMA/CD
Because CSMA allows each computer to determine whether
a shared cable is already in use by another computer, it prevents a computer
from interrupting an ongoing transmission.
However, CSMA cannot prevent all possible conflicts. To understand why, imagine what happens if
two computers at opposite ends of an idle cable both have a frame ready to send
at the same time. When they check for a carrier, both stations. Find the cable
idle, and both start lo send frames simultaneously. The signals travel at
approximately 70% of the speed of light, and when the signals transmitted by
two computers reach the same point on the cable, they interfere with each
other.
The interference between two signals is called a
collision. Although a collision does not harm the hardware, it produces a
garbled transmission that prevents either of the two frames from being received
correctly. To ensure that no other
computer transmits simultaneously, the Ethernet standard requires a sending
station to monitor signals on the cable. If the signal on the cable differs
from the signal that the station is sending, it means that a collision has
occurred. Whenever a collision is
detected, a sending station immediately stops transmitting. Technically,
monitoring a cable during transmission is known as Collision Detect {CD), and
the Ethernet mechanism is known as Carrier Sense Multiple Access with Collision
Detect (CSMA/CD).
CSMA/CD does more than merely detect collisions - it
also recovers from them. After a collision occurs, a computer must wail for the
cable to become idle again before transmitting a frame. However, if the
computers begin to transmit as soon as the ether becomes idle another collision
will occur. To avoid multiple collisions, Ethernet requires each computer to
delay after a collision before attempting to retransmit. The standard specifies a maximum delay, d,
and forces each computer to choose a random delay less than d. In most cases, when a computer chooses a
delay at random, it will select a value that differs from any of the values
chosen by the other computers – the computer that chooses the smallest delay
will proceed to send a frame and the network will return to normal operation.
If two or more computers happen to choose nearly the
same amount of delay after a collision, they will both begin to transmit at
nearly the same time, producing a second collision. To avoid a sequence of
collisions, Ethernet requires each computer to double the range from which a
delay is chosen after each collision. Thus, a computer chooses a random delay
from 0 to d after one collision, a random delay between 0 and 2d after a second
collision, between 0 and 4d after a third, and soon after a few collisions, the
range from which a random value is chosen becomes large, and the probability is
high that some computer will choose a short delay and transmit without a
collision.
Technically, doubling the range of the random delay
after each collision is known as binary exponential back off. In essence, exponential back off means that
an Ethernet can recover quickly after a collision because each computer agrees
to wait longer times between attempts when the cable becomes busy. In the
unlikely event that two or more computers choose delays that are approximately
equal, exponential back off guarantees that contention for the cable will be
reduced after a few collisions.
Computers attached to an Ethernet use CSMA/CD in which
a computer waits for the ether lo be idle before transmitting a frame. If two computers transmit simultaneously, a
collision occurs: the computers use exponential back off to choose which
computer will proceed. Each computer' delays a random time before trying to
transmit again, and then doubles the delay for each successive collision.
Basis
and Working
Ethernet is a very popular local area network
architecture based on the CSMA/CD access method. The original Ethernet
specification was the basis for the IEEE 802.3 specifications. In present
usage, the term "Ethernet" refers to original Ethernet (or Ethernet
II, the latest version) as well as the IEEE 802.3 standards. The different
varieties of Ethernet networks are commonly referred to as Ethernet topologies.
Typically, Ethernet networks can use a bus physical topology, although, as
mentioned earlier, many varieties of Ethernet such as 10BASE-T use a star
physical topology and a bus logical topology. (Microsoft uses the term
"star bus topology" to describe 10BASE-T.)
Ethernet networks, depending on the specification,
operate at 10 or 100Mbps using base band transmission. Each IEEE 802.3
specification prescribes its own cable types.
OSI
Model
This
model is based on a proposal developed by the International Standards
Organization (ISO) as a first step toward international standardization of the
protocols used in the various layers. The model is called the ISO-OSI (Open
Systems Interconnection) Reference Model because it deals with connecting open
systems—that is, systems that are open for communication with other systems. We
will usually just call it the OSI model for short.
The OSI model has seven layers. The
principles that were applied to arrive at the seven layers are as follows
1. A layer should be created where a different level of abstraction
is needed.
2. Each layer should perform a well-defined function.
3. The function of each layer should be chosen with an eye toward
defining internationally standardized protocols.
4. The layer boundaries should be chosen to minimize the information
flow across the interfaces.
5. The number of layers should be large enough that distinct
functions need not be thrown together in the same layer out of necessity, and
small enough that the architecture does not become unwieldy.
Below we will discuss each layer of the
model in turn, starting at the bottom layer. Note that the OSI model itself is
not network architecture because it does not specify the exact services and
protocols to be used in each layer. It just tells what each layer should do.
However, ISO has also produced standards for all the layers, although these are
not part of the reference model itself.
Each one has been published as a separate international standard.
Figure (16) - The OSI Reference Model
The Physical Layer
The physical layer is concerned with transmitting raw
bits over a communication channel. The design issues have to do with making
sure that when one side sends a 1 bit, it is received by the other side as a 1
bit, not as a 0 bit. Typical questions here are how many volts should be used
to represent a 1 and how many for a 0, how many microseconds a bit lasts,
whether transmission may proceed simultaneously in both directions, how the
initial connection is established and how it is torn down when both sides are
finished, and how many pins the network connector has and what each pin is used
for. The design issues here largely deal with mechanical, electrical, and
procedural interfaces, and the physical transmission medium, which lies below
the physical layer.
The
Data Link Layer
The main task of the data link layer is to take a raw
transmission facility and transform it into a line that appears free of
undetected transmission errors to the network layer. It accomplishes this task
by having the sender break the input data up into data frames (typically a few
hundred or a few thousand bytes), transmit the frames sequentially, and process
the acknowledgement frames sent back by the receiver. Since the physical layer
merely accepts and transmits a stream of bits without any regard to meaning or
structure, it is up to the data link layer to create and recognize frame
boundaries. This can be accomplished by attaching special bit patterns to the
beginning and end of the frame. If these bit patterns can accidentally occur in
the data, special care must be taken to make sure these patterns are not
incorrectly interpreted as frame delimiters.
A noise burst on the line can destroy a frame
completely. In this case, the data link layer software on the source machine
can retransmit the frame. However,
multiple transmissions of the same frame introduce the possibility of duplicate
frames. A duplicate frame could be sent if the acknowledgement frame from the
receiver back to the sender were lost.
It is up to this layer to solve the problems caused by damaged, lost,
and duplicate frames. The data link
layer may offer several different service classes to the network layer, each of
a different quality and with a different price.
Another issue that arises in the data link layer (and
most of the higher layers is well) is how to keep a fast transmitter from
drowning a slow receiver in data. Some traffic regulation mechanism must be
employed to let the transmitter know how much buffer space the receiver has at
the moment. Frequently, this flow regulation and the error handling are
integrated.
If the line can be used to transmit data in both
directions, this introduces a new complication that the data link layer
software must deal with. The problem is that the acknowledgement frames for A
to B traffic compete for the use of the line with data frames for the B to A
traffic.
Broadcast networks have an additional issue in the data
link layer to control access to the shared channel. A special, sub layer of the
data link layer, the medium access sub layer, deals with this problem.
The
Network Layer
The network layer is concerned with controlling the
operation of the subnet. A key design issue is determining how packets are
routed from source to destination.
Routes can be based on static tables that are "wired into" the
network and rarely changed. They can also be determined at the start of each
conversation, for example a terminal session. Finally, they can be highly
dynamic, being determined anew for each packet, to reflect the current network
load.
If too many packets are present in the subnet at the
same time, they will get in each other's way, forming bottlenecks. The control of such congestion also belongs
to the network layer.
Since the operators of the subnet may well expect
remuneration for their efforts, there is often some accounting function built
into the network layer. At the very
least, the software must count how many packets or each customer sends
characters or bits, to produce billing information. When a packet crosses a
national border, with different rates on each side, the accounting can become
complicated.
When a packet has to travel from one network to another
to get to its destination, many problems can arise. The addressing used by the second network may
be different from the first one. The second one may not accept the packet at
all because it is too large. The protocols may differ, and so on. It is up to
the network layer to overcome all these problems to allow heterogeneous
networks to be interconnected.
In broadcast networks, the routing problem is simple,
so the network layer is often thin or even nonexistent.
The
Transport Layer
The basic function of the transport layer is to accept
data from the session layer, split it up into smaller units if need be, pass
these to the network layer, and ensure that the pieces all arrive correctly at
the other end. Furthermore, all this must be done efficiently, and in a way
that isolates the upper layers from the inevitable changes in the hardware
technology.
Under normal conditions, the transport layer creates a
distinct network connection for each transport connection required by the
session layer. If the transport connection requires a high throughput, however,
the transport layer might create multiple network connections, dividing the data
among the network connections to improve throughput. On the other hand, if
creating or maintaining a network connection is expensive, the transport layer
might multiplex several transport connections onto the same network connection
to reduce the cost. In all cases, the transport layer is required to make the
multiplexing transparent to the session layer.
The transport layer also determines what type of
service to provide the session layer, and ultimately, the users of the network.
The most popular type of transport connection is an error-free point-to-point
channel that delivers messages or bytes in the order in which they were
sent. However, other possible kinds of
transport service are transport of isolated messages with no guarantee about
the order of delivery, and broadcasting of messages to multiple destinations.
The type of service is determined when the connection is established.
The transport layer is a true end-to-end layer, from
source to destination, in other words, a program on the source machine carries
on a conversation with a similar program on the destination machine, using the
message headers and control messages. In
the lower layers, the protocols are between each machine and its immediate
neighbors, and not by the ultimate source and destination machines, which may
be separated by many routers. There is a difference between layers 1 through 3,
which are chained, and layers 4 through 7, which are end-to-end. Many hosts are
multi-programmed, which implies that multiple connections will be entering and
leaving each host. Their needs to be some way to tell which message belong to
which connection. The transport header is one place this information can be
put.
In addition to multiplexing several message streams
onto one channel, the transport layer must take care of establishing and
deleting connections across the network. This requires some kind of naming
mechanism, so that a process on one machine has a way of describing with whom
it wishes to converse. There must also be a mechanism to regulate the flow of
information, so that a fast host cannot overrun a slow one. Such a mechanism is
called flow control and plays a key role in the transport layer (also in other
layers). Flow control between hosts is
distinct from flow control between routers, although we will later see that
similar principles apply to both.
The
Session Layer
The session layer allows users on different machines to
establish sessions between them. A session allows ordinary data transport, as
does the transport layer, but it also provides enhanced services useful in some
applications. A session might be used to
allow a user to log into a remote timesharing system or to transfer a file
between two machines.
One of the services of the session layer is to manage
dialogue control. Sessions can allow traffic to go in both directions at the
same time, or in only one direction at a time. If traffic can only go one way
at a time (analogous to a single railroad track), the session layer can help
keep track of whose turn it is.
A related session service is token management. For some protocols, it is essential that both
sides do not attempt the same operation at the same time. To manage these
activities, the session layer provides tokens that can be exchanged. Only the
side holding the token may perform the critical operation.
Another session service is synchronization. Consider
the problems that might occur when trying to do a 2-hour file transfer between
two machines with a 1-hour mean time between crashes. After each transfer was
aborted, the whole transfer would have to start over again and would probably
fail again the next time as well. To eliminate this problem, the session layer
provides a way to insert checkpoints into the data stream, so that after a
crash, only the data transferred after the last checkpoint have to be repeated.
The
Presentation Layer
The presentation layer performs certain functions that
are requested sufficiently often to warrant finding a general solution for
them, rather than letting each user solve the problems. In particular, unlike
all the lower layers, which are just interested in moving bits reliably from
here to there, the presentation layer is concerned with the syntax and
semantics of the information transmitted.
A typical example of a presentation service is encoding
data in a standard agreed upon way. Most user programs do not exchange random
binary bit strings. They exchange things such as people's names, dates, amounts
of money, and invoices. These items are represented as character strings,
integers, floating-point numbers, and data structures composed of several
simpler items. Different computers have different codes for representing
character strings, integers, and so on. In order to make it possible for
computers with different representations to communicate, the data structures to
be exchanged can be defined in an abstract way, along with a standard encoding
to be used "on the wire." The presentation layer manages these
abstract data structures and converts from the representation used inside the computer
to the network standard representation and back.
The
Application Layer
The application layer contains a variety of protocols
that are commonly needed. For example, there are hundreds of incompatible
terminal types in the world. Consider, the plight of a full screen editor that
is supposed to work over a network with many different terminal types, each
with different screen layouts, escape sequences for inserting and deleting
text, involving the cursor, etc.
One way to solve this problem is to define an abstract
network virtual terminal that editors and other programs can be written to deal
with. To handle each terminal type, a piece of software must be written to map
the functions of the network virtual terminal onto the real terminal. For example,
when the editor moves the virtual terminal's cursor to the upper left-hand
corner of the screen, this software must issue the proper command sequence to
the real terminal to get its cursor there too. All the virtual terminal
software is in the application layer.
Another application layer function is file transfer.
Different file systems have different file naming conventions, different ways
of representing text lines, and so on. Transferring a file between two
different systems requires handling these and other incompatibilities. This
work, too, belongs to the application layer, as do electronic mail, remote job
entry, directory lookup, and various other general purpose and special-purpose
facilities.
Let us now turn from the OSI reference model to the
reference model used in the grandparent of all computer networks, the ARPANET,
and its successor, the worldwide Internet. Although we will give a brief
history of the ARPANET later, it is useful to mention a few key aspects of it
now. The ARPANET was a research network
sponsored by the DOD (U.S Department of Defense). It eventually connected
hundreds of universities and government installations using leased telephone
lines. When satellite and radio networks were added later, the existing protocols
had trouble interworking with them, so new reference architecture was needed.
Thus the ability to connect multiple networks together in a seamless way was
one of the major design goals from the very beginning. This architecture later
became known as the TCP/IP Reference Model, after its two primary protocols.
Given the DOD's worry that some of its precious hosts,
routers, and internet work gateways might get blown to pieces at a moment's
notice, another major goal was that the network be able to survive loss of
subnet hardware, with existing conversations not being broken off. In other
words, DOD wanted connections to remain intact as long as the source and
destination machines were functioning, even if some of the machines or
transmission lines in between were suddenly put out of operation. Furthermore,
a flexible architecture was needed, since applications with divergent
requirements were envisioned, ranging from transferring files to real-time
speech transmission.
The
Internet Layer
All these requirements led to the choice of a
packet-switching network based on a connectionless Internet work layer. This
layer, called the Internet layer, is the linchpin that holds the whole
architecture together. Its job is to permit hosts to inject packets into any network
and have them travel independently to the destination (potentially on a
different network). They may even arrive
in a different order than they were sent, in which case it is the job of higher
layers to rearrange them, if in-order delivery is desired. Note that
"internet" is used here in a generic sense, even though this layer is
present in the Internet.
The analogy here is with the mail system. A person can
drop a sequence of international letters into a mailbox in one country, and
with a little luck, most of them will be delivered to the correct address in
the destination country. Probably the
letters will travel through one or more international mail gateways along the
way, but this is transparent to the users. Furthermore, that each country has
its own stamps, preferred envelope sizes, and delivery rules is hidden from the
users. The Internet layer defines an official packet format and protocol called
IP (Internet Protocol). The job of the Internet layer is to deliver IP packets
where they are supposed to go. Packet routing is clearly the major issue here,
as is avoiding congestion. For these reasons, it is reasonable to say that the
TCP/IP Internet layer is very similar in functionality to the OSI network
layer. Figure 2.15 shows this
correspondence.
Figure (17) -
The TCP/IP Reference Model
The
Transport Layer
The layer above the Internet layer in the TCP/IP model
is now usually called the transport layer. It is designed to allow peer
entities on the source and destination hosts to carry on a conversation, the
same as in the OSI transport layer. Two end-to-end protocols have been defined
here. The first one, TCP (Transmission Control Protocol) is a reliable
connection-oriented protocol that allows a byte stream originating on one
machine to be delivered without error on any other machine in the Internet. It
fragments the incoming byte stream into discrete messages and passes each one
onto the Internet layer. At the destination, the receiving TCP process reassembles
the received messages into the output stream. TCP also handles flow control to
make sure a fast sender cannot swamp a slow receiver with more messages than it
can handle.
The second protocol in this layer, UDP (User Data gram
Protocol), is an unreliable, connectionless protocol for. Applications that do
not want TCP's sequencing or flow control and wish to provide their own. It is
also widely used for one-shot, client-server type request-reply queries and
applications in which prompt delivery is more important than accurate delivery,
such as transmitting speech or video. The relation of IP, TCP, and UDP . Since
the model was developed, IP has been implemented on many other networks.
Figure(18) - Protocols and Networks in the TCP/IP Model
Initially
The
Application Layer
The TCP/IP model does not have session or presentation
layers. No need for them was 'perceived, so they were not included. Experience
with the OSI model has proven this view correct: they are of little use to most
applications.
On top of the transport layer is the application layer.
It contains all the Higher Level Protocols. The early ones included virtual
terminal (TELNET), file transfer (FTP), and electronic mail (SMTP). The virtual
terminal protocol allows a user on one machine to log into a distant machine
and work there. The file transfer protocol provides a way to move data
efficiently from one machine to another. Electronic mail was originally just a
kind of file transfer, but later a specialized protocol was developed for it. Many
other proto- cols have been added to these over the years, such as the Domain
Name Service (DNS) for mapping host names onto their network addresses, NNTP,
the protocol used for moving news articles around, and HTTP, the protocol used
for fetching pages on the World Wide, and many others.
The
Host-to-Network Layer
Below the Internet layer is a great void. The TCP/IP
reference model does not really say much about what happens here, except to
point out that the host has to connect to the network using some protocol so it
can send IP packets over it. This protocol is not defined and varies from host
to host and network to network. Books and papers about the TCP/IP model rarely
discuss it.
Ethernet
Cabling
The types of Ethernet cables
available are
1.
Straight-through
cable
2.
Crossover
cable
3.
Rolled
cable
Straight-through cable
Four wires are used in
straight-through cable to connect Ethernet devices. It is relatively simple to
create this type. Only pins1, 2, 3 and 6 are used. Just connect 1 to1, 2 to 2,
3 to 3 and 6 to 6 and you will be up and networking in no time while
practically we connect all 4 pairs straighten of CAT-5. However, this would be
an Ethernet only cable and would not work with Voice, Token Ring, ISDN, etc.
This type of cable is used to connect
1.
Host
to switch or hub
2.
Router
to switch or hub
Figure (19) - Straight-through cable
Crossover Cable
Four wires
are used in straight-through cable to connect Ethernet devices. Only four pins
are used in this type of cabling. In crossover cable we connect 1 to3
and 2 to 6 on each side of cable. This
type of cable is used to connect
1.
Switch
to switch
2.
Hub
to hub
3.
Host
to host
4.
Hub
to switch
5.
Router
direct to host
Figure (20) - Cross over cable
Rolled Cable
Although rolled cable is not
used to connect any Ethernet connections together, you can use a rolled
Ethernet cable to connect a host to a router console serial communication (com)
port. If you have a Cisco router of switch, you would use this cable to connect
your PC running Hyper Terminal to the Cisco hardware. Eight wires are used in
this cable to connect serial devices, although not all eight are used to send
information, just as in Ethernet networking
Figure (21)
- Rolled cable
Ethernet
Addressing
MEDIA ACCESS CONTROL ADDRESS
Ethernet addressing uses Media
Access Control (MAC) Address burned into each and every Ethernet Network
Interface Card (NIC). The MAC or hardware address, is a 48-bit (6-byte) address
written in a hexadecimal format.
24
Bits
24 Bits
47 46
I/G G/L Originally
Unique Vender Assigned
Identifier
(OUI)
Figure (22) – Ethernet addressing
using MAC addresses
The organizationally unique identifier (OUI)
is assigned by the IEEE to an organization. It’s composed of 24 bits, or 3
bytes. The organization, in turn, assigns a globally administered address (24
bits, or 3 bytes) that is unique (supposedly, again-no guarantees) to each and
every adapter they manufacture. The high-order bit is the individual/Group
(I/G) bit. When it has a value of 0, we can assume that the address is MAC
address of a device and May well appear in the source portion of the MAC
header. When it is a 1, we can assume that the address represents either a
broadcast or multicast address in Ethernet, or a broadcast .The next bit is the
G/L bit (also known as U/L, where U means universal). When set to 0,this bit
represents a globally administered address (as by IEEE). When the bit is 1, it
represents a locally governed and administered address (as in DECnet). The
low-order 24 bits of an Ethernet address represent a locally administered or
manufacturer-assigned code. This portion commonly starts with 24 0s for the
first card made and continues in order until there are 24 1s for the last card
made. You will find that many manufacturers use these same six hex digits as
the last six characters of their serial number on the same card.
IP ADDRESSING
An IP address is a numeric
identifier assigned to each machine on an IP network. It designates the
specific location of a device on the network.
An IP address is a software
address, not a hardware address- the latter is hard-coded on a Network
Interface Card (NIC) and used for finding hosts on a local network. IP
addressing was designed to allow a host on one network to communicate with a
host on a different network, regardless of the type of LANs the hosts are
participating in.
There are two IP addressing schemes:
- Hierarchical IP addressing
- Private IP Addressing
Hierarchical IP addressing
An IP address consists
of 32 bits of information. These bits are divided into four sections, referred
to as octets or bytes, each containing 1 byte (8 bits). You can depict an IP
address using one of three methods:
- Dotted-decimal, as in 172.16.30.56
- Binary, as in 10101100.00010000.00011110.00111000
- Hexadecimal, as in AC.10.1E.38
All these examples
represent same IP address. The 32-bit IP address is a structured or hierarchical
address, as opposed to a flat or nonhierarchical address. Although either type
of addressing scheme could have been used, hierarchical addressing was chosen
for a good reason. The advantage of this scheme is that it can handle a large
number of addresses, namely 4.3 billion. The disadvantage of the flat
addressing scheme, and the reason it’s not used for IP addressing, relates to
routing. If every address were unique, all routers on the Internet would need
to store the address of each and every machine on the Internet. This would make
efficient routing impossible, even if only a fraction of the possible addresses
were used.
The solution to this problem is to
use a two or three-level, hierarchical addressing scheme that is structured by
network and host, or network, subnet, and host.
This two- or three-level scheme is
comparable to a telephone number. The first section, the area code, designates
a very large area. The second section, the prefix, narrows the scope to a local
calling area. The final segment, the customer number, zooms in on the specific
connection. IP address uses the same type of layered structure. Rather than all
32 bits being treated as a unique identifier, as in flat addressing, a part of
the address is designated as the network address, and the other part is
designated as either the subnet and host or just the node address.
NETWORK ADDRESSING
The network address (which can also
be called the network number) uniquely identifies each network. Every machine
on the same network shares that network address as part of its IP address. In
the IP address 172.16.30.56, for example, 172.16 is the network address.
The
nodes address is assigned to, and uniquely identifies, each machine on a
network. This part of the address must be unique because it identifies a
particular machine-an individual as opposed to a network, which is a group.
This number can also be referred to as a host address. In the sample IP address
172.16.30.56 is the node address.
The
designers of the Internet decided to create classes of networks based on
network size. For the small number of networks possessing a very large number
of nodes, they created the rank Class ‘A’ network. At the other extreme
is the Class ‘C’ network, which is reserved for the numerous networks
with a small, is predictably called the Class ‘B’ network.
Subdividing
an IP address into a network and node address is determined by the class
designation of one’s network
8 Bits 8 Bits 8 Bits 8 Bits
 |
Class ‘A’ : Network Host Host Host
8 Bits 8 Bits 8 Bits 8 Bits
|
Class ‘B’ : Network Network Host Host
8 Bits 8 Bits 8 Bits 8 Bits
|
Class ‘C’ : Network Network Network Host
Class
‘D’ : Multicast
Class
‘E’ : Research
Figure
(23) - Summary of the three classes of Networks
To ensure efficient routing,
Internet designers defined a mandate for the leading-bits section of the
address for each different network class. For example, since a router knows
that a Class ‘A’ network address always starts with a 0, the router might be
able to speed a packet on its way after reading only the first bit of its
address. This is where the address schemes define the difference between a
Class ‘A’, a Class ‘B’, and a Class ‘C’ address. In the next section, I will
discuss the differences between these three classes, followed by a discussion
of the Class ‘D’ and Class ‘E’ address.
Network Address
Range - Class ‘A’
The designers of
the IP address scheme said that the first bit of the first byte in a Class ‘A’
network address must always be off, or 0. This means a Class ‘A’ address must
be between 0 and 127. So a Class ‘A’ network is defined in the first octet
between 0 and 127, and it can’t be less or more.
Network Address
Range - Class ‘B’
In a Class ‘B’
network, the RFCs state that the first bit of the first byte must always be
turned on, but the second bit must always be turned off. If you turn the other
6 bits all off and then all on, you will find the range for a Class ‘B’
network, thus a Class ‘B’ network is defined when the first byte is configured
from 128 to191.
Network Address range - Class ‘C’
The first
three bytes of a Class ‘C’ network address are dedicated to the network portion
of the address, with only one measly byte remaining for the node address. Thus
a class ‘C’ network is defined when first byte is configured from192 to 223.
Private IP Addresses
These addresses
can be used on a private network, but they are not routable through the
Internet. This is designed for the purpose of creating a measure of well-needed
security, but it also conveniently saves valuable IP address space.
If every host on every network had to have real routable IP address, we
would have run out of IP address to hand out years ago. But by using private IP
address, ISPs, corporation, and home users only need a relatively tiny group of
bona fide IP addresses to connect their networks to the Internet. This is
economical because they can use private IP addresses on their inside networks
and get along just fine.
To accomplish this task, the
ISP and the corporation-the end user, no matter who they are-need to use
something called a Network Address Translation (NAT), which basically takes a
private and converts it for use on the Internet. Many people can use the same
real IP address to transmit out onto the Internet.
APPLICATIONS
There is a long list of application areas, which can be
benefited by establishing Computer Networks. Few of the potential applications
of Computer Networks are:
1.
Information
retrieval systems which search for books, technical reports, papers and articles
on particular topics
2.
News access
machines, which can search past news, stories or abstracts with given search
criteria.
3.
Airline
reservation, hotel booking, railway-reservation, car-rental, etc.
4.
A writer's aid: a
dictionary, thesaurus, phrase generator, indexed dictionary of quotations, and
encyclopedia.
5.
Stock market
information systems which allow searches for stocks that meet certain criteria,
performance comparisons, moving averages, and various forecasting techniques.
6.
Electronic
Financial Transactions (EFT) between banks and via cheque clearing house.
7.
Games of the type
that grow or change with various enthusiasts adding to the complexity or
diversity.
8.
Electronic Mail
Messages Systems (EMMS).
9.
Corporate
information systems such as marketing information system, customer information
system, product information system, personnel information system, etc.
10.
Corporate systems
of different systems such as Order-Entry System, Centralized Purchasing,
Distributed Inventory Control, etc.
11.
On-line systems for
Investment Advice and Management, Tax Minimization, etc.
12.
Resources of
interest to a home user.
13.
Sports results.
14.
Theatre, movies,
and community events information.
15.
Shopping
information, prices, and advertisements.
16.
Restaurants; good
food guide.
17.
Household magazine,
recipes, book reviews, film reviews.
18.
Holidays, hotels,
travel booking.
19.
Radio and TV
programmes.
20.
Medical assistance
service.
21.
Insurance
information.
22.
Computer Assisted
Instruction (CAI).
23.
School homework,
quizzes, tests.
24.
Message sending
service.
25.
Directories.
26.
Consumer reports.
27.
Employment
directories and Job opportunities.
28.
Tax information and
Tax assistance.
29.
Journey planning
assistance viz. Train, bus, plane etc.
30.
Catalogue of Open
University and Virtual
University courses.
CONCLUSION
After completion of this practical
training I conclude that training in the Engineering is an essential task for
each and every student and it must be taken seriously.
The
practical training is a chance to the trainee to gain the practical knowledge
in the field and to work in the scheduled environment. Apart from these things it improves the
managing qualities, which are essential for an engineer.
It is very
fruitful experience for me to overcome my weaknesses and how to face and solve
the difficulties arises in the field.
REFERENCES
1) Computer Networks By Andrew S.
Tanenbaum.
2) Computer Networks By William
Stalling.
3) Wireless Communication &
Networking By William Stalling.
4) CCNA Study Guide By BPB
Publications.
